Sodium pyrithione (also called the sodium salt of 1-hydroxy-2-pyridinethione, sodium pyridine-2-thiol-N-oxide, or 2-pyridinethiol-1-oxide, Na salt) has excellent antimicrobial properties, and is a well-known commercial product typically employed as a biocide and preservative in functional fluids, such as metalworking fluids, lubricants, cosmetics and toiletries. Sodium pyrithione is commonly made by reacting 2-chloropyridine-N-oxide with NaSH and NaOH, as disclosed, for example, in U.S. Pat. No. 3,159,640.
Likewise, zinc pyrithione [also known as zinc pyridine-2-thiol-N-oxide or bis [1-hydroxy-2(H) pyridinethionato]-zinc] is a broad-spectrum antimicrobial agent and preservative in metalworking fluids, plastics, paints, adhesives and cosmetics. One of the principal applications for zinc pyrithione is its use as an anti-dandruff agent in shampoo.
Zinc pyrithione is typically made at the sodium pyrithione production plant by reacting sodium pyrithione with a zinc salt, such as zinc sulfate, to form a zinc pyrithione precipitate, as disclosed, for example in U.S. Pat. No. 2,809,971, which is typically washed and then dispersed in water to form an aqueous dispersion of zinc pyrithione in water. This conversion of sodium pyrithione to form the zinc pyrithione dispersion at the pyrithione manufacturing plant is labor intensive. Moreover, the resulting zinc pyrithione dispersion is subject to settling problems, inasmuch as the zinc pyrithione tends to physically settle out of the dispersion during shipment or storage prior to use of the dispersion. This settling problem necessitates mixing of the dispersion by the manufacturer of personal care compositions containing the dispersion in order to insure homogenity of the dispersion in the composition. In addition, the settling problem can result in "caking" of the zinc pyrithione in the bottom of the drum in which it is stored and shipped, particularly when this product is stored for several months or subjected to wide fluctuations in ambient temperature.
In view of the above, it should be clear that there is a need for improved methodology for preparing zinc pyrithione-containing personal care compositions without risking the zinc pyrithione dispersion settling problems encountered heretofore. The present invention provides one solution to this need.